1. Field of the Invention
The present invention relates to a photomask of a semiconductor device. More particularly, the present invention relates to a technique of exposing a resist with electrons and to a method of manufacturing a photomask using such a technique.
2. Description of the Related Art
An electron beam exposure process is typically used to manufacture photomasks. In this process, a layer of a predetermined material is formed over the entire surface of a substrate, the layer of material is coated with an electron beam resist, and then the resist is irradiated with an electron beam to form a desired pattern. The irradiated electron beam resist is developed to form an electron beam resist pattern. Then, the underlying layer of material is etched using the electron beam resist pattern as a mask, whereby a photomask having a desired pattern is produced.
As increases in the integration density of semiconductor devices are sought, the design rule for the devices becomes smaller. Thus, a critical dimension of the pattern of a photomask used in manufacturing semiconductor devices is also becoming smaller. Accordingly, efforts are being made to enhance the resolution of the electron beam-exposing device that is used to irradiate electron beam resists. These efforts have concentrated on raising the accelerating voltage of the electron beam in an attempt to reduce the range over which incident electrons are scattered. However, using an electron beam produced using a high accelerating voltage creates an unexpected problem. This problem will now be described with reference to FIGS. 1(A) and 1(B).
FIG. 1(A) illustrates the behavior of an electron beam 103 incident upon a photomask substrate 100 having a common opaque layer 101 and an electron beam resist 102. The hatched portion 106 of the electron beam resist 102 is to be exposed. Electrons 104 incident upon the electron beam resist scatter at various points in the electron beam resist, i.e. in the midst of the resist and at the interface between the resist 106 and opaque layer 102, resulting in decreases in their energy. Some of the energy of the scattered electrons is absorbed in a region of the electron beam resist 102 outside of the region 106 to be exposed.
FIG. 1(B) illustrates a distribution of the absorption energy, which distribution results from the scattering of the incident electrons. The incident electrons of an electron beam having a low accelerating voltage 111 are scattered over a wide range. Thus, the absorption energy is also distributed over a wide region. However, the electrons of an electron beam having a high accelerating voltage 112 are scattered over a very narrow range. Although the absorption energy is distributed over a very narrow region, the electrons tend to penetrate the photomask substrate 100 completely rather than being scattered in the electron beam resist. Therefore, only a small proportion of the total energy of the beam is absorbed at the electron beam resist. For example, a typical dose of radiation provided by an exposing device having an accelerating voltage of 10 keV is 8 xcexcC. However, in comparison, an exposing device having an accelerating voltage of 50 keV must provide a dose of radiation of 32 xcexcC and over a much longer exposing time to produce the same amount of absorbed energy in a comparable resist than the exposing device having an accelerating voltage of 10 keV. Therefore, although photomasks having small critical dimensions can be manufactured by an exposing device that generates electron beams having a high accelerating voltage, the use of such a device compromises the productivity of the manufacturing process.
An object of the present invention is to solve the above-described problem by providing a technique by which a pattern of a photomask having a high degree of resolution can be produced in a short time.
To achieve the above object, the present invention provides a method in which an electron beam resist is exposed (patterned) with electron beams having low and high accelerating voltages, respectively. The step of exposing the electron beam resist to the electron beam having the low accelerating voltage causes the resist to absorb an amount of energy less than the threshold energy of the resist. The energy is nonetheless absorbed quickly because the electrons do not penetrate the resist completely so as to scatter. The electron beam resist is exposed with the electron beam having a high accelerating voltage to provide the high degree of resolution of the pattern formed on the resist. The electron beam resist can be exposed to the electron beams in any order.
This technique can be applied directly to the method of manufacturing a photomask, according to the present invention. In this case, a phase shift film and/or an opaque layer is formed on a transparent substrate, and the electron beam resist is coated on the phase shift film and/or the opaque layer. The electron beam resist is then exposed by a first electron beam and by a second electron beam having a higher accelerating voltage than that of the first electron beam, whereby a desired pattern is formed. The electron beam resist is then developed to produce an electron beam resist pattern. Next, the phase shift film and/or the opaque layer are etched using the electron beam resist pattern as an etching mask, whereby a photomask having a desired pattern is manufactured.
The technique can also be applied indirectly to the method of manufacturing a photomask, according to the present invention. Specifically, several test electron beam resists are produced, and a test pattern is reproduced on each of the test electron beam resists using the above-describe technique. The test electron beam resists are then developed to form test electron beam resist patterns. Data representative of characteristics of the test electron beam resist patterns is then generated. For instance, a critical dimension of each of the electron beam resist patterns is measured.
Appropriate dosages of the first and second electron beams for use in patterning the resist of the photomask are selected, based on the data, from among those used to form the test electron beam resist patterns.